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Tumour necrosis factor-alpha regulates expression of the CCAAT-enhancer-binding proteins (C/EBPs) alpha and beta and determines the occupation of the C/EBP site in the promoter of the insulin-responsive glucose-transporter gene in 3T3-L1 adipocytes.

ABSTRACT: We have demonstrated previously that treatment of 3T3-L1 adipocytes with tumour necrosis factor-alpha (TNF) results in a rapid (4 h) and significant (75-80%) reduction in the rate of transcription of the GLUT4 gene. Control of GLUT4 gene transcription has been suggested at least in part to reside with the CCAAT-enhancer-binding protein (C/EBP) family (alpha, beta and delta isoforms) of transcription factors. Using electrophoretic mobility shift assays, we have examined the ability of TNF to alter the occupation of the C/EBP site in the GLUT4 promoter. The data suggest that in fully differentiated adipocytes the C/EBP site is a ligand for predominantly alpha/alpha homodimers; however, after exposure to TNF, a shift in occupancy of the site occurs and the ligands become alpha/beta heterodimers and beta/beta homodimers. Partner selection in dimer formation appears to be controlled by selective translocation of the beta-isoform from the cytosol to the nucleus after exposure of the cells to TNF.

SUBMITTER: Jain R

PROVIDER: S-EPMC1220111 | biostudies-other | 1999 Mar

REPOSITORIES: biostudies-other

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Project description:In obesity, high levels of tumor necrosis factor ? (TNF?) stimulate lipolysis in adipocytes, leading to hyperlipidemia and insulin resistance. Thiazolidinediones (TZDs), the insulin-sensitizing drugs, antagonize TNF?-induced lipolysis in adipocytes, thereby increasing insulin sensitivity in diabetes patients. The cellular target of TZDs is peroxisome proliferator-activated receptor ? (PPAR?), a nuclear receptor that controls many adipocyte functions. As a transcription factor, PPAR? is closely modulated by coregulators, which include coactivators and corepressors. Previous studies have revealed that in macrophages, the insulin-sensitizing effect of PPAR? may involve suppression of proinflammatory gene expression by recruiting the corepressor complex that contains corepressors and histone deacetylases (HDACs). Therefore, we investigated whether the corepressor complex is involved in TZD-mediated suppression of TNF?-induced lipolysis in 3T3-L1 adipocytes. Trichostatin A (TSA), a pan HDAC inhibitor (HDACI) that inhibits class I and II HDACs, was used to examine the involvement of HDACs in the actions of TZDs. TSA alone increased basal lipolysis and attenuated TZD-mediated suppression of TNF?-induced lipolysis. Increased basal lipolysis may in part result from class I HDAC inhibition because selective class I HDACI treatment had similar results. However, attenuation of TZD-mediated TNF? antagonism may be specific to TSA and related hydroxamate-based HDACI rather than to HDAC inhibition. Consistently, corepressor depletion did not affect TZD-mediated suppression. Interestingly, TSA treatment greatly reduced PPAR? levels in differentiated adipocytes. Finally, extracellular signal-related kinase 1/2 (ERK1/2) mediated TNF?-induced lipolysis, and TZDs suppressed TNF?-induced ERK phosphorylation. We determined that TSA increased basal ERK phosphorylation, and attenuated TZD-mediated suppression of TNF?-induced ERK phosphorylation, consistent with TSA's effects on lipolysis. These studies suggest that TSA, through down-regulating PPAR?, attenuates TZD-mediated suppression of TNF?-induced ERK phosphorylation and lipolysis in adipocytes.

Dataset Information

Tumour necrosis factor-alpha regulates expression of the CCAAT-enhancer-binding proteins (C/EBPs) alpha and beta and determines the occupation of the C/EBP site in the promoter of the insulin-responsive glucose-transporter gene in 3T3-L1 adipocytes.

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